Un errore comune è parlare troppo in fretta.

Because errore is a masculine singular noun that begins with a vowel, so Italian uses un before it.

• un errore = a mistake / an error
• uno is used before certain consonant sounds, such as:
  • uno studente
  • uno zaino
  • uno psicologo

So errore takes un, not uno.

In Italian, adjectives often come after the noun, especially when they are simply describing it in a neutral way.

So:

• un errore comune = a common mistake

This is the most natural order here.
Italian can sometimes place adjectives before the noun, but that often changes the tone or emphasis. In this sentence, errore comune is the standard, natural combination.

The accent shows that è means is.

• è = is
• e = and

This is a very important spelling difference in Italian.
So in this sentence:

• Un errore comune è... = A common mistake is...

Without the accent, it would be a different word.

Because after è, Italian is defining what the mistake is, and it uses the infinitive to do that.

So:

• Un errore comune è parlare troppo in fretta.
• literally: A common mistake is to speak too fast.

This works much like English to speak.

Other similar examples:

• Il problema è capire. = The problem is understanding / to understand.
• La cosa migliore è aspettare. = The best thing is to wait.

So parlare is used because the sentence is saying what the mistake consists of.

Not in this sentence.

• parlare = to speak
• parlando = speaking

After è, when you are naming an action in a general way, Italian normally uses the infinitive:

• Un errore comune è parlare troppo in fretta.

Using parlando here would sound wrong because parlando is a gerund, and that is used in different structures, such as:

• Sto parlando. = I am speaking.
• Impari parlando. = You learn by speaking.

So here, parlare is the correct form.

Because troppo modifies the whole idea of in fretta.

• in fretta = quickly / in a hurry
• troppo in fretta = too quickly / too fast

So troppo comes right before the expression it is modifying.

Compare:

• parlare in fretta = to speak quickly
• parlare troppo in fretta = to speak too quickly

This is very natural word order in Italian.

In fretta is an expression meaning quickly, in a hurry, or fast depending on context.

In this sentence:

• parlare troppo in fretta = to speak too fast / too quickly

It is a fixed expression:

• Sono in fretta. = I’m in a hurry.
• Fallo in fretta. = Do it quickly.

So even though fretta by itself means hurry, the expression in fretta often functions like an adverb.

Yes, you could say:

• Un errore comune è parlare troppo velocemente.

That is grammatically correct and means essentially the same thing. However, troppo in fretta often sounds more natural and conversational in everyday Italian.

A rough difference:

• troppo in fretta = very common, natural, idiomatic
• troppo velocemente = also correct, a bit more direct or formal-sounding in some contexts

Both work, but in fretta is very common in speech.

Because Italian infinitives do not need a separate word like English to.

• parlare already means to speak
• mangiare = to eat
• studiare = to study

So in Italian, the infinitive itself includes the idea of to + verb.

That is why:

• è parlare = is to speak

There is no extra word needed.

They are very close, and in many situations both can mean mistake.

• errore = error, mistake
• sbaglio = mistake

In this sentence, both are possible:

• Un errore comune è parlare troppo in fretta.
• Uno sbaglio comune è parlare troppo in fretta.

But there is a grammar difference in the article:

• un errore
• uno sbaglio

because sbaglio begins with sb-, which takes uno.

In tone, errore can sound slightly more formal or neutral, while sbaglio can feel a bit more everyday in some contexts.

Because the subject is already there:

• Un errore comune = a common mistake

That whole noun phrase is the subject of the sentence.

So the structure is:

• Un errore comune = subject
• è = is
• parlare troppo in fretta = what the subject is being equated with

Italian does not need an extra dummy subject like English it in this kind of sentence.

Yes. That is a very natural English translation.

The Italian structure allows English translations such as:

• A common mistake is to speak too fast.
• One common mistake is speaking too fast.

Both reflect the meaning well. The Italian sentence itself is neutral and natural, and English can express it in more than one way.

You would say:

• Errori comuni sono parlare troppo in fretta e non ascoltare abbastanza.

But more naturally, if you are making a full plural sentence, Italian often prefers a different structure, for example:

• Due errori comuni sono parlare troppo in fretta e non ascoltare abbastanza.
  = Two common mistakes are speaking too fast and not listening enough.

Or:

• Gli errori comuni includono il parlare troppo in fretta.
  = Common mistakes include speaking too fast.

The singular sentence you were given is very straightforward because it introduces one example:

• Un errore comune è... = A common mistake is...